I'm no expert on extending Construct, but I wouldn't be surprised if it would be possible for users to create their own plugin/extension for specific services if they are so inclined to do so. Rex has been a hero in that regard for C2.

As I've seen many people uses OwnCloud or NextCloud. It would be nice if C3 supported those as well. But as things are currently standing the sub-folder and iCloud support is top priority in that regard imho.